docs: def fold_up(self, f_inter, f_leaf):
          """
          Apply recursively the function f_inter from leaves to root, begining with function f_leaf on leaves.
          
          """
          return f_leaf(self) if self.leafnode else f_inter(self,
                                                            self.lower.fold_up(f_inter, f_leaf), 
                                                            self.greater.fold_up(f_inter, f_leaf))